Fluid flow through orifices is common. Examples include fuel injector nozzle tips, carburetor jets, cooling air flow through turbine engine components, lubricating oil metering for precision bearings and the like. In many such applications, metering of flow rates is of very great importance, but due to manufacturing artifacts, is of very great difficulty. Even very slight variations in manufacturing tolerances can produce substantial variations in flow resistance and flow.
Parts having fluid flow orifices are made by a wide variety of casting and machining procedures. For example, high quality investment castings are frequently employed for manufacture of such parts. Such parts will have variations in dimensions, particularly wall thicknesses attributable to slight core misalignments or core shifting, and other variations in surface conditions, including surface roughness, pits, nicks, gouges, blow holes, or positive metal. In the extreme, a very slight crack in a core can lead to a thin wall projecting into an internal passage. All these artifacts will substantially impede fluid flow.
Commonly employed machining .methods, such as conventional drilling, electrical discharge machining and even less usual techniques as laser, electron beam, electrostream, and STEM drilling are not sufficiently precise to avoid the generation of substantial variations in flow resistance. Probably, the most precise of these, electrical discharge machining, will not produce perfectly uniform flow resistance since the length of an internal passage may vary as an incident of casting operations, giving rise to fluctuations of hole length and flow resistance despite the uniformity of the hole diameter. In addition, non-uniform EDM conditions are inevitable and may produce variations in size, shape, surface finish and hole edge conditions.
At present, such deviations are necessarily tolerated with broad limits and the attendant compromises in design freedom, performance, and efficiency are accepted as unavoidable. For example, the delivery of fuel charges to internal combustion engines by pressurized fuel injection requires metering of flow through injectors. The more precisely the flow can be regulated, the greater will be the fuel efficiency and economy of the engine operation. At present, the design of such fuel metering systems is often based on measurement of actual flow resistance and segregation of inventories within ranges of flow parameters to provide at least approximate matching of components in inventory within a range of deviation from defined tolerances. Such operations are a considerable expense because of the substantial inventory requirements. In addition, a substantial number of components must be rejected as out of allowable deviations and must be reworked at considerable expense or discarded.
At the present time, injector nozzles are machined with the critical flow metering orifices formed by electrical discharge machining. As shown in FIG. 1, the most critical flow resistance determinants are considered to be the diameter of orifices 10, 11, and wall thickness at section line A--A, as well as edge condition and surface roughness, including "lay" of the finish. The design specifications are for a wall thickness at this point of 0.040 inches.+-.0.002 inches. Parts outside these specifications are rejected. Accepted parts are segregated in inventory into eight ranges, .+-.0.00025 inches. Those of ordinary skill in the art have long been aware that wall thickness at A--A is an indirect determinant of flow resistance of orifices 10, 11, and that the accurate control of the diameter of orifices 10, 11, is a direct determinant of flow resistance. These parameters determine flow metering properties, and a more direct measure of flow resistance of the part and a direct control in manufacture of such flow resistance is highly desirable.
Another example of flow resistance through an orifice of significant criticality is the provision of cooling air flow through gas turbine engine components, such as turbine blades. As shown in FIGS. 3 and 4, investment cast turbine blades are typically cast or drilled (by laser drilling, STEM drilling, or electrical discharge machining), to provide a plurality of holes, typically having a nominal diameter of about 0.010 to 0.030 inches, passing from the interior passage to the vicinity of the leading edge, trailing edge, and elsewhere along the airfoil. Cooling air is forced from the interior, out the plural holes, and into the high temperature combustion gas stream to provide cooling of the blade. Sometimes holes through internal walls of the blade meter distribution of cooling air. It is reasonably apparent that variations in flow resistance can result in different cooling effects which can result in hot spots which may alter the heat balance within the components and the engine itself and affect both performance and component life. Cooling air usage should however be minimized as its excessive usage reduces engine efficiency by "stealing" compressor section energy. More precise control of flow resistance of these orifices can provide substantial benefits in operation of such components and of the units into which they are assembled.
There are numerous other examples of flow control orifices to which the present invention is applicable, but the foregoing examples show representative examples which serve to illustrate the state of the art and the problem addressed and solved by the present invention.
It is an object of the present invention to provide a method of working flow controlling orifices in structures to attain a precise flow resistance.
It is a further object to provide such a method to provide a precise predetermined flow resistance.
Another object is to provide such a method where by a plurality of flow controlling orifices can be provided with closely matched flow resistance.
Still another object is to provide a method whereby a continuing plurality of parts can be made having predetermined flow resistance with high levels of precision and reproducibility.
Yet another object is to provide a method of attaining a predetermined flow resistance through an orifice by controlling the operation through a parameter which is a direct determinant of or a direct consequent of flow resistance.
A further object is to provide parts and components having orifices with reproducible, precise, predetermined flow resistances.
Still another object is to provide "tuned" turbine engine components.